Partial coherence interferometers measure distances by monitoring interference as a function of wavelength. A measuring beam, which has a spectral bandwidth spanning a range of wavelengths, is temporarily split into a test beam and a reference beam. The test beam propagates along a test arm encountering a test surface en route to a detector. The reference beam propagates along a reference arm encountering a reference surface en route to the same detector. Height variations between different points on the test surface produce corresponding changes in the optical path length of the test arm.
The detector separates the combined test and reference beams into their common spectral components and detects varying intensities associated with the interference for each sampled wavelength. The interference as a function of wavelength is related to optical path length differences between the test and reference beams. A so-called “null condition” occurs when the optical path lengths of the test and reference arms are equal. The period of the interference maximums and minimums increases on opposite sides of the null condition independently of whether the test arm is longer than the reference arm or the reference arm is longer than the test arm.
Thus, partial coherence interferometers are generally arranged to avoid the null condition, but if operating in the vicinity of the null condition, the partial coherence interferometers must also be arranged to distinguish between optical path lengths of the test arm that are longer than or shorter than the optical path length of the reference arm. Range sensors, particularly triangulation range sensors, have been used for identifying limited ranges of unambiguous measurements. However, these range sensors are generally inadequate for covering larger ranges of measurement with high resolution and are more disturbed by irregular surfaces including surfaces with tool marks or other artifacts of roughness, i.e., surfaces having peak to valley roughness of at least ⅛ wavelength of the measuring beam.